Women may not benefit from repeated frozen embryo transfers: a retrospective analysis of the cumulative live birth rate of 43 972 women.

study questionWhich specific groups of women would not benefit from repeated frozen embryo transfers (FETs)? SUMMARY ANSWER: Women over 45 years of age should stop treatment after three FET attempts due to the absence of further benefits, while women aged 40-45 years and those with a diminished ovarian reserve and other causes of infertility have a lower chance of improving their cumulative live birth rate (CLBR) within five FET cycles and experience fewer advantages from repeated transfers. WHAT IS KNOWN ALREADY: In real-life scenarios of ART, women who fail to achieve a live birth often choose to undergo repeated FETs via the freeze-all strategy. STUDY DESIGN SIZE DURATION: This retrospective study included 43 972 women who underwent 86 496 oocyte retrieval cycles and 82 022 FET cycles between January 2010 and March 2023 under the freeze-all strategy. PARTICIPANTS/MATERIALS SETTING

methodsWe categorized the population based on the female's age at the first oocyte pick-up (OPU) cycle (Groups 1-6: <30, 30-34, 35-39, 40-42, 43-44, and ≥45 years of age), number of retrieved oocytes at the first OPU cycle (Groups 1-5: 1-5, 6-10, 11-15, 16-20, and >20 oocytes), and causes of infertility (Groups 1-9: tubal factor, male factor, polycystic ovary syndrome, diminished ovarian reserve, endometriosis, other uterine factors, combined factors, unexplained infertility, and other infertility) to analyse their CLBRs within different FET cycles via Kaplan-Meier analysis (optimistic method) and the competing risk method (conservative method). We utilized multivariate Cox and Fine-Gray models to examine the associations between the CLBR and age, the number of retrieved oocytes, and nine causes of infertility. MAIN RESULTS AND THE ROLE OF CHANCE: The CLBR decreased with increasing female age over five FET cycles (Groups 1-6: optimistic method: 96.4%, 94.2%, 86.0%, 50.2%, 23.1%, and 10.1%; conservative method: 87.1%, 82.0%, 67.8%, 33.9%, 13.8%, and 3.5%, respectively). Moreover, there was an increasing trend in the number of retrieved oocytes (Groups 1-5: optimistic method: 82.5%, 91.7%, 93.6%, 94.1%, and 96.2%; conservative method: 58.6%, 76.7%, 84.8%, 88.0%, and 92.5%, respectively). Furthermore, the CLBR varied across different causes of infertility (Groups 1-9: optimistic method: 91.7%, 93.1%, 96.6%, 79.2%, 89.9%, 76.1%, 90.0%, 92.9%, and 35.4%; conservative method: 77.3%, 79.4%, 88.9%, 46.7%, 72.7%, 62.1%, 74.4%, 78.8%, and 20.1%, respectively). LIMITATIONS REASONS FOR CAUTION: Calculating the actual CLBR for each person is difficult because some patients have remaining embryos that have not been transferred; additionally, the current statistical methodology uses both optimistic and conservative methods to calculate the CLBR, and in real life, the CLBR falls between the optimistic and conservative curves. WIDER IMPLICATIONS OF THE

findingsOur study is the first to identify specific subgroups of women who fail to benefit from repeated FETs and who require rational discontinuation of treatment following unsuccessful transfer. STUDY FUNDING/COMPETING INTERESTS: This study was financially supported by grants from the National Natural Science Foundation of China (grant numbers: 82271732 to Y.K., 82071603 to L.W., 82001502 to Y.L., and 82201888 to X.S.). The authors declare that they have no conflicts of interest in the present study. TRIAL REGISTRATION NUMBER: N/A.